I think you probably have a situation where some kids in the neighborhood did a little smash and grab.
I think education and awareness are the two best things for you to pursue here.Use this as a chance to talk to them about how dangerous the real world is, and that they need to be more aware and vigilant.
Start preaching the basics:
Lock the doors, dont leave valuables in sight, dont open the door to strangers, dont leave the garage door opener in sight, parking lot safety, etc. etc.....